Nearly four hundred years ago, in a patchwork of individual fiefdoms
that we now call Italy, a revolution of ideas was struggling to take
place. The traditional way to understand the workings of the
world -- through a combination of divine revelation and abstract
reasoning --
had begun to come under attack from a new breed. These people called
themselves “natural philosophers,” because the word “scientist” had not
yet been invented. To find out the way the world worked, they didn’t
sit around and talk about it. They went out and looked. This was not an
approach that was likely to find favor with the Church, home of
received wisdom, or with its
instruments -- the whispering Inquisitors, with their hotline back to
Rome. Now, a certain natural philosopher had fallen very foul of those
Inquisitors and been forced to stop his investigations into the
structure of the heavens. His name was Galileo Galilei, and our story
begins with him.

Convent of Minerva, Rome
June 22, 1633

I, Galileo Galilei, son of the late
Vincenzo Galilei, Florentine,
aged seventy years, arraigned personally before this tribunal, and
kneeling before you, most Eminent and Reverend Lord Cardinals,
Inquisitors general against heretical depravity throughout the whole
Christian Republic . . . have been pronounced by the Holy Office to be
vehemently suspected of heresy, that is to say, of having held and
believed that the sun is the center of the world and immovable, and
that the earth is not the center and moves:

Therefore, desiring to remove from the
minds of your Eminences, and
of all faithful Christians, this strong suspicion, reasonably conceived
against me, with sincere heart and unfeigned faith I abjure, curse, and
detest the aforesaid errors and heresies . . . and I swear that in the
future I will never again say or assert, verbally or in writing,
anything that might furnish occasion for a similar suspicion regarding
me.

As the great Galileo rose from his knees at the end of this
infamous, and forced, recantation, he is said to have muttered “Eppur
si muove!” (“And yet it moves!”). He knew in his heart that
Earth moves
around the sun, in spite of what the Inquisitors had made him say.
Still, devoutly religious as he was, he had no taste for defying his
own church. Nor had he any desire to share the fate of the unfortunate
monk Giordano Bruno, who a few decades earlier had been publicly burned
for holding similar views. Galileo may have been the most famous
philosopher in all Italy, but he knew that in itself wouldn’t save him
from the fire.

And though he was now seventy years old, frail, and steadily losing
his sight, he was not yet ready to die. He had damaged his eyes by
staring through a telescope at wonders he himself had discovered:
blemishes that appeared periodically on the surface of the sun; craters
on the moon; distant but distinct moons circling the planet Jupiter
(who would have thought that other planets could have moons of their
own?), and stars that nobody knew existed. Now, before the cataracts
and glaucoma finally clouded his sight, in secret, if necessary, he had
one last task to complete. Galileo had seen this “trial” coming; he’d
known for some time that he couldn’t continue his study of the heavens.
So for some years he had been discreetly changing tack, turning his
attention inwards to Earth itself. And, failing eyesight
notwithstanding, he was about to change the way we see the most
apparently ordinary substance in the world: air.

The Inquisitors knew nothing of this. They were satisfied with his
recantation, and decided, graciously, to spare his life. He would be
allowed to return to his villa at Arcetri in Florence, though he should
understand that he was still considered dangerous and would therefore
be held under house arrest. There would be no visitors, save those
given prior permission by the Church. Meanwhile, Galileo himself was to
spend his time reciting the holy psalms as penance, and praying for his
immortal soul.

Galileo returned to his villa as instructed and performed his
penance diligently. But the Inquisitors had also obliged him to swear
never again to publish work that might offend the Holy Office, and he
had no intention of complying. For with him to Arcetri he had taken a
certain manuscript that was already nearly finished.

He had started the experiments it described while awaiting his
summons to Rome. Having turned away from his telescope, Galileo had
become fascinated instead by the different ways that objects move
through the air. The result was to become his masterpiece. The
manuscript already recounted findings that would become just as famous
as the moons of Jupiter. For instance, Galileo had made the surprising
discovery that Earth’s gravity doesn’t care in the least how much
something weighs. Drop a cannonball and a pebble from a high tower, and
both will reach the ground at exactly the same moment.

But within its pages was another discovery that would prove to be
less famous yet no less significant. Galileo had measured the weight of
air.

This might seem like a bizarre notion. How can something so
insubstantial as the air weigh anything at all? In fact our planet’s
air is constantly pushing down on us with great force. We don’t notice
this because we’re used to it, like lobsters sauntering along on the
seafloor, unaware of the crushing weight of the ocean of water above
them. We give our own overlying air-ocean so little respect that we
even describe anything that’s full of air as being “empty.”

Back in Galileo’s time, notions about air were similarly hazy. Most
people accepted the idea put forward by Aristotle in the fourth century
b.c. that everything in the world was made up of four elements: earth,
air, fire, and water. Earth and water were obviously pulled downward by
gravity. Fire was obviously weightless. But air was the problem child.
Was it heavy enough to be dragged to the ground, light enough to rise
like flames do, or did it simply ignore Earth’s gravitational tug and
hover?

Galileo believed that air is heavy and had set about testing his
idea. The experiments he performed were typically ingenious. First, he
took a large glass bottle with a narrow neck and a tight leather
stopper. Into this stopper he inserted a syringe attached to a bellows
and by working vigorously managed to squeeze two or three times more
air into the bottle than it had previously contained. Next, he weighed
the glass bottle most precisely, adding and subtracting the finest of
sand to his scales until he was satisfied with the answer. Then, he
opened a valve in the lid. Immediately, the compressed air rushed out
of its confinement, and the bottle was suddenly a handful of grains
lighter. The air that had escaped must account for the missing weight.

This showed that air is not the insubstantial body we usually take
it for. But now Galileo wanted to know how much air corresponded to how
many grains of sand. For that he would somehow need to measure both the
weight of the escaping air and its volume.

This time, he took the same glass bottle with its long, narrow neck.
However, instead of pumping it full of extra air, he forced in some
water. When the bottle was three-quarters full of water, its original
air was squeezed uncomfortably into a quarter of its original space.
Galileo weighed the bottle accurately, opened the valve, allowed this
pressurized air to escape, and then weighed the bottle again to find
out how much air he had lost. As for the volume, Galileo reasoned that
the portion of air that had been forced to leave the bottle had been
pushed aside by the water he had squeezed in, so the volume of air that
had fled must be exactly the same as the volume of water that remained.
All he had to do was pour out the water and measure its volume and
voilà, he had found the weight
for a given volume of air.

The value Galileo came up with was surprisingly large: Air seemed to
weigh as much as one four-hundredth the weight of an equivalent amount
of water. If that doesn’t sound like much, consider this. Picture a
particular volume of air for a
moment -- such as the “empty” space inside Carnegie Hall in New York.
How heavy would you expect that amount of air to be? Would it weigh ten
pounds? Or a hundred? Or maybe even five hundred?

The answer is somewhere in the region of seventy thousand pounds.

The weight of air is so extreme that even Galileo didn’t see the
whole story. He never considered the question of how we can shoulder
such a crushing, overwhelming burden, for the simple reason that he
didn’t realize the air above us
is still heavy. He had measured the
weight of air in his bottle, but he was convinced that the moment this
air was released back into its natural element, the sky, it immediately
ceased to weigh anything at all.

Galileo believed that our atmosphere as a whole is incapable of
pushing. It was one of the few occasions when the great man was wrong.

In spite of the Church’s opposition Galileo finished his manuscript
-- and published it. After fruitless efforts to convince publishers in
Florence, Rome, and Venice to defy the Inquisitors, Galileo finally
smuggled the manuscript out to a printer in the Netherlands. Four years
later, as he approached the end of his life, a few copies began
filtering back to Italy. Each bore a disingenuous disclaimer by Galileo
himself, who wrote how astonished he was that his words had somehow
found their way to a printer’s in spite of his obedience to the Papal
diktat.

And although Galileo was wrong about the way our air behaves aloft,
the experiments his great work contained would influence two very
different people to discover the truth.